The present invention relates to an electron multiplier device comprising microchannel plates with optical feedback suppression which can be used, for example, in image intensifier tubes.
Hereinafter, "optical feedback" is to be understood to mean the light which is emitted by the screen of an image intensifier tube, in reaction to the detection of an electron, and which passes to the interior of the tube to reach the photocathode. Optical feedback of this kind can excite the photocathode and can start a reaction process which leads to overdriving and destruction of the tube. The effect of this feedback is a problem when the intensifier operates with a very high gain such as, for example, when use is made of an imaging technique involving the counting of photons when the gain is on the order of 10.sup.7 photons emitted by the screen for each photon detected (or photoelectron released) by the cathode.
For the contruction of image intensifier tubes, various arrangements are known which aim to reduce such optical feedback. Thus, the inner side of the screen is provided with a metallic diaphragm, generally made of aluminium, for the outward reflection of the light emitted to the interior of the tube by the screen. However, this diaphragm comprises microholes. Also, the opacity of the diaphragm, instead of being absolute, even the best of cases is not better than on the order of 10.sup.5. As a result, optical feedback is not eliminated.
Another arrangement concerns the use of microchannel plates as electron multipliers. The light emitted to the tube interior necessarily passes through the microchannels where it is at least in part subjected to numerous reflections from the walls. Each reflection is accompanied by absorption of light. This results in an attenuation of the light passing through the microchannel plate which is estimated to be on the order of 100 in the case of a plate with straight channels and on the order of 1000 in the case of a plate comprising curved channels of the kind described in French Patent Specification No. 2,168,861. The optical feedback which still persists at the entrance of the plate corresponds essentially to the light emitted by the screen in directions near to the directions of the channel axis at the exit of the plate.
A more effective known arrangement involves the use of an electron multiplier in the form of a "chevron" assembly of two superposed plates comprising straight channels as described in U.S. Pat. No. 3,374,380. Actually, while the light emitted by the screen in the direction of the axis of a straight channel passes therethrough without being reflected from the walls and thus without attenuation, this light is necessarily subjected to reflections in the channels of a chevron assembly of two channels. The number of reflections in such an assembly and the attenuation is larger as the angle enclosed by the axes of the channels is made larger. The maximum permissible value of this angle is limited, this value being linked to the limits which may not be exceeded by the inclinations of the normals to the faces of the channel plate with the axes of the channels of the two plates. At the entrance of the first plate, the inclination limit is in the order of 10.degree. in order to ensure suitable detection of the photoelectrons. At the exit of the second plate, the inclination limit has the same order of magnitude in order to prevent the image spot from becoming elliptical, which would lead to a loss of resolution in the direction of the main axis of the ellipse and which would be difficult to handle by data processing systems situated on the tube and employing, for example, a Plumbicon (trademark) brand camera tube and a system for digitizing the signal as regards amplitude and position.